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The below URL is a general format that has been implemented in other environments and is an out of the box solution for AX retrieval.
Servername = The name of the server where AX web is setup
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Add additional Indexfield using the & symbol and enter necessary values for retrieval.
I've mocked up the url below further to emphasize the changes needed to make the URL work.
Above I specified an additional index field number. In my example field5 is a doctype and Invoice is the specific doctype to match with the previous Invoice number (field1) of 123456.